Quality Control

Quality Control at Coral Plast

The importance of injection moulding process monitoring QC in injection moulding involves the systematic monitoring and evaluation of the manufacturing process to ensure that the final product meets the desired quality standards.

Our quality control specialist check the following parameters used for injection moulding process monitoring and qc checking inside and outside the machine.

1. Tolerance Level

This is all about the permissible limits within which the dimensions of a moulded product can vary from its original design.They are expressed as a plus or minus value.Tolerances tend to range from +/- 0.1mm for general parts or +/-0.025mm for components requiring high precision manufacturing.

2. Cavity Pressure

This is measured using sensors to ensure optimal production at various stages in manufacturing process. This aspect of injection moulding process monitoring helps prevents faults such as burn marks and other aesthetic defects being caused by the cavity pressure being too low or too high during production.

3. Time, Speed and Rate of Injection

Being able to monitor performance by comparing the time,speed and rate of injection with industry standards for the type of project under assessment helps quality control monitors spot any anomalies that could lead to faults in the product.

4. Cooling Time

In most injection moulding manufacturing projects,cooling times take up most of the production cycle time. Ensuring the correct time is factored into production timetables can greatly enhance the quality of the finished part.

5. Adherence to original customer specification

As with any manufacturing or any other type of project – referring back to original customer specifications is the key for quality monitoring purposes. Having a clear list of expected specs to compare against the final component will throw light on any deviations very quickly.

Eight problems commonly found during injection moulding process monitoring:

While the hope is that the vast majority of products will pass the stringent quality control checks – if not all of them, there will sometimes be cases where mistakes are made and some products are rejected during the monitoring process.

1. Sink Marks

These are very common defects that happen when depressions(craters,dents) appear in the thicker areas of the component being molded. They are very small, but can cause issues with aesthetics and performance.

2. Burn or gas Marks

Burn marks also known as gas marks take on a rusty colour and appear on the surface of injection moulded products. They can be caused by excessive heating or overly fast injection speeds both of which should be picked up and fixed during injection molding process monitoring.

3. Short shots

This happens when insufficient plastic reaches the mould leaving some parts empty and incomplete or when air gets trapped and cannot escape, so takes the place of some of the plastic inside the mould.

4. Flow lines

These are lines or streaks that appear on the surface of the part often in a different colour. They are caused by varying speeds at which the molten material flows through the machine and adheres to the shape of the mould.

5. Weld Lines

Weld lines show up at areas where molten plastic meets each other after flowing in from different parts of the mould. They are a sign of weak bonding which can lead to problems with the structural integrity of the finished part.

6. Scratches

The scratches on the appearance of an injection molded product usually occur on such deep cavity products.
When it comes to the scratches on the textured surface, the first thing that comes to the mind of an engineer is that the draft angle on the side of the product is not large enough.
Excessive injection pressure and holding pressure are also important that causes scratches on product side walls. Excessive injection pressure creates stress that causes product warpage in the mould. The product may be locally pressed on the side wall so that it is easy to scratch the product side surface during mold ejection.

7. Flash

Flash, also known as flashing or burrs, mostly occurs between the matching parts of the mould, the sliding parts of the slider, the clearance of the insert and the gap of the ejector etc. If the flashing is not resolved in a timely manner, it will be further worsened so that the mould will be pressed to cause local collapses or other permanent damages. The flashing occurring in the clearance of the insert and the gap of the ejector will also cause the product to stick on the mould thus affecting ejection

8. Black spots can be due to

  • Incoming resin
  • Degradation of the resin in the screw
  • Degradation of the resin in the hot runner
  • Broken or poorly designed check valves and screw tips.
  • Poorly designed nozzle body.
  • Mismatched junctions
  • Poor thermocouple placements or poor temperature control.
  • Contamination
  • Long residence time for using less than 25 % barrel.
  • Excessive melt temperature.

 

Minimizing Black spots:

  • If you suspect the barrel and screw, which are often the culprits, clean the screw and barrel. Pull the screw and check for carbon build up behind the flights in the metering and transition zones of the screw.
  • Cleaning should be done with dry ice or high pressure jet. Most general purpose screws produce black specks due to dead space behind the flights. Specially a screw with no dead spots in the flights starts with the last flight of the feed section. It should look something like a farmer plough.
  • Do not use a barrier screwor high injection mixing device.
  • Avoid vented barrels
  • Often the screw and vent design are prone to dead spots with resulting degradation to carbon.
  • The screw should be highly polished with no scratches .
  • The flights should be sharp, scratch free with no marks.

 

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